a. Field of the Invention
This invention relates to transmissions, particularly those adapted for use in ultralight aircraft, air boats and homebuilt aircraft, which provide a reduction between the propulsion source output shaft rotation rate and the propeller rotation rate.
b. Brief Description of the Prior Art
In many applications, such as ultralight aircraft which use internal combustion engines, the engine rotation rate is greater, in normal operation, than the desired rotation rate for the driven load, such as the propeller of the aircraft. In ultralight aircraft, the propeller generally rotates at a relatively slow speed and is of comparatively great length. Therefore, in conventional practice, direct drive gear reduction transmissions have been utilized to reduce the propeller rotation rate to about one half of the engine revolution speed. Such transmissions conventionally have two gears, a drive gear which is driven directly by the internal combustion engine output shaft, and an output gear, which is driven by the drive gear. In such an application, the output gear is laterally offset from the drive gear and is axially aligned with the axis of rotation of the propeller. Thus, the propeller axis of rotation is offset from the axis of rotation of the power source drive shaft.
Transmissions of the conventional type, such as that described, have several disadvantages when utilized in ultralight aircraft. As is obvious, the transmission must be of extremely light weight in order to be commercially practical. Thus, the gears are generally of comparatively lightweight construction. The principal problem which is encountered in such transmissions is a result of the condition known as "torque effect", which is exhibited by an airframe in response to the rotation of the aircraft propeller. Torque effect is described, for example, at pages 33 and 34 of "Flight Training Handbook", revised 1980, published by the United States Department of Transportation, Federal Aviation Administration.
The two principal factors in torque effect of concern in ultralight aircraft are the "gyroscopic precession" effect and the "P-factor" effect. In the gyroscopic precession effect, the rotation of the propeller causes a force to be transmitted through the transmission, the engine, and the engine mounts, to the airframe to cause the aircraft to tend to yaw. In the event that the propeller axis of rotation is not aligned with the axis of rotation of the power source drive shaft, the gyroscopic precession effect exerts an additional force on the gears and transmission case which necessitates additional structural rigidity in order to transmit the force through the transmission to the engine without component failure.
The P-factor effect presents a different problem. By reason of the normal use of a two bladed propeller in ultralight aircraft, the P-factor results in a bending moment being imposed upon the transmission components and cover, which varies from zero to a maximum twice during each propeller revolution. The bending moment may result in failure of the gear teeth, the drive shafts, or the transmission case during operation in the conventional type of transmission, which uses direct drive between the gears and an offset axis of rotation of the propeller with respect the drive shaft. In order to avoid such failure, additional structural rigidity, by way of mass for the transmission components, must be provided, thereby increasing the weight of the transmission and thus the aircraft itself.